Add test 'TestCoverageOfImplicitPermutationsIsComplete' to set some expectations
for the implicit permutation in synthesis API - the latter not being implemented
yet.
Correct test expectations to take sub-permutations into account.
Another correction to the test expectations - must take into account the actual
number of test variables in the fine slice.
Always choose test variables from different subtrees to ensure the resulting
test cases actually exhibit permutation when sliced.
Make the generation of permutations a bit less eager.
Only one subtree in an interleave has to be forced to contain a permuting
synthesizing node.
Implement functionality tested by
'TestCoverageOfImplicitPermutationsIsComplete'.
Optimise test performance for 'TestCoverageOfImplicitPermutationsIsComplete' and
fix bug where the permutation example was being produced for a non-prunable
subtree of a pruned tree root.